非结构性碳水化合物和水分关系如何调节热带干性和湿性森林树木的径向生长？

Tropical forest is a crucial component of the global carbon cycle. Elevated atmospheric CO2 concentration could enhanced tropical forest tree growth through increasing leaf photosynthetic capacity. However, studies from both long term forest plot and tree ring revealed non-significant change in tropical forest tree growth. To predict the impact of elevated CO2 on tropical tree growth, the key question needed to be answered is where and when the tree growth is limited by carbon related to photosynthesis rate and /or limited by other factors such that water status. We will choose an evergreen tree species (Pometia tomentosa) and a deciduous tree species (Toona ciliata) co-occurring in tropical karst forest (dry forest) and tropical seasonal forest (wet forest) in Xishuangbanna, and monitor the seasonal dynamics of cambial activity, non-structural carbohydrates (NSCs) concentrations of different organs, water relations, as well as the environmental factors. Our aims are to investigate 1) the differences in the seasonal growth patterns of evergreen and deciduous trees in different tropical forest with contrasting soil water status, 2) the contribution of water relations and NSCs to the seasonal growth of tropical forest trees and 3) how did environmental factors affect water relations and NSCs, as well as the growth rates of tropical trees. Understanding the seasonal pattern of tropical forest tree growth and the eco-physiological regulation mechanism is crucial for predicting the impact of CO2 fertilization effect on tropical tree growth and evaluating the carbon fixation capacity of tropical forest under global climate change.

热带森林是全球碳循环的重要组成部分。大气CO2浓度升高可能通过提高光合能力进而加速热带树木生长。然而基于长期固定森林样地和树木年轮的研究显示，CO2施肥效应对热带森林树木生长的促进作用并不明显。预测CO2升高对热带森林的影响，关键要从机理上弄清热带树木的径向生长是如何受碳可利用性和水分关系调控的。本项目拟在西双版纳地区水分条件迥异的两种热带森林中选择共有的常绿树种和落叶树种，通过监测其形成层活动、不同部位非结构性碳水化合物（NSCs）含量、水分关系以及环境因子的季节动态变化，弄清1）不同生境和功能群热带树种形成层活动和径向生长的季节动态；2）树木NSCs和水分关系的季节动态及其与径向生长的关联；3）环境因子如何通过影响树木的生理过程进而影响径向生长。阐明热带树木生长的季节动态及其生理调控机制，有助于正确理解CO2浓度施肥效应对热带树木生长的影响和评估气候变化背景下热带森林的固碳能力。

热带森林是全球碳循环的重要组成部分，理解热带树木的径向生长季节动态及其对环境因子的响应，有助于预测气候变化背景下热带树木的固碳能力。本项目选取西双版纳热带季节雨林和热带喀斯特森林的典型树种，监测了其径向生长和形成层活动季节动态，测定了枝条、叶片和树干边材非结构性碳水化合物的季节动态变化，探讨了树木径向生长和形成层活动季节动态及其与环境因子的关系。本项目还探讨了热带季节雨林与喀斯特森林功能性状的差异及其与径向生长的关系，并分析了热带地区部分树种径向生长-气候关系。主要发现：1）热带季节雨林和喀斯特森林树木的径向生长均主要受到水分状况的影响，但热带季节雨林的树木具有相对较高的年生长量和日生长速率；2019年的极端干旱，影响了喀斯特生境树木的木质部发育和径向生长。2）发现红椿、多花白头树、番龙眼、梭果玉蕊、麻楝等树种的茎干边材的非结构性碳水化合物并没有显著的季节变化，而红椿和番龙眼的小枝淀粉含量呈现出生长季前高、生长季开始之后降低、生长季末期又升高的趋势。3）与热带季节雨林的植物相比，喀斯特森林植物叶片具有较为保守的水分利用策略，但其叶片具有较高的磷含量和较高的光合速率；热带季节雨林树木径向生长速率与功能性状之间的关系受到径级的影响。4）发现热带和亚热带地区红椿、南酸枣、云南松和思茅松等树木的径向生长年际波动主要受干季和生长季早期水分有效性的限制。本项目的研究结果可为正确评估热带森林对气候变化的响应提供理论依据。